As far as I know this tiara was not from Queen Marie's mother, the Duchess of Eimburgh, even though Queen Marie inherited her mother's gorgeous pearls, as you said.
I think I read that it was bought from Cartier, like the long diamonds sautoir with the huge sapphire pendant of 478 cts.
In fact, I think it was actually Lisabeta, the Queen of Greece that inherited this pearls & diamonds tiara, while Mignon, the Queen of Yugoslavia got her grand-mother's diamonds fringe tiara, and Princess Ileana, Archduches of Austria-Tuscany got the sapphires & diamonds kokoshnik tiara.